Torque stabilizer for brushless servo motor

ABSTRACT

When it is judged that a driven body reaches vicinity of a target position, a present phase of the passing current is not switched to keep a state of the passing current even if a phase of a passing current needs to be switched judging from the position of permanent magnet rotor thereby a stabilizing of torque can be attained.

TECHNICAL FIELD

The present invention relates to a device for stabilizing torque of abrushless servo motor used as a driving source such as a throttle valve,an actuator, an exhaust gas recirculation (hereinafter referred to asEGR) valve and the like.

BACKGROUND ART

FIG. 1 is an explanatory diagram in which a valve 11 of the EGR valvearranged in an exhaust gas return passage (c) for making an exhaust gaspassage (a) of an engine E communicate with an intake gas passage (b)thereof is an object to be controlled and in which a brushless servomotor 21 is driven by an output of a control section 100 to control anopening of the valve 11.

FIG. 2 is a longitudinal cross sectional view to show a constitution ofthe EGR valve having the brushless servo motor 21 as a driving source towhich a servo control device is applied. In FIG. 2, a reference numeral1 denotes a valve body in which a passage communicating with the exhaustgas return passage (c) disposed in a recirculation system of an exhaustgas is formed. When the valve 11 is moved up as shown in the drawing, itis put into contact with a seat 12 to thereby close the exhaust gasreturn passage (c) and when the valve 11 is moved down, it is separatedfrom the seat 12 to thereby open the exhaust gas return passage (c).

A reference numeral 21 denotes a brushless servo motor to be a drivingmeans (torque generating source) of a valve for opening/closing theexhaust gas passage, 22 denotes a motor case mounted on the valve body1, 23 denotes a rotor rotatably supported in the motor case 22 viabearings 24, 25, and the rotor 23 has a screw hole 23 a which is madethrough a central portion of a shaft. A reference numeral 26 denotes amagnet mounted on an outer peripheral portion of the rotor 3, 27 denotesa stator core surrounding an outer periphery of the magnet 26, andbetween this stator core 27 and the above mentioned magnet 26 is formeda predetermined gap.

A reference numeral 28 denotes a coil that is arranged on the statorcore 27 and it constitutes a stator, 29 denotes a motor shaft to be avalve driving shaft, and this motor shaft 29 is made as a screw shaftand screwed into the screw hole 23 a of the above mentioned rotor 23 andmade to move in an axial direction by a rotation of the above mentionedrotor 23.

A reference numeral 30 denotes a valve shaft abutting member integrallyformed with the motor shaft 29. By making a top end of a valve shaft 13having the above mentioned valve 11 at its top end abut against the tip(bottom end in FIG. 1) of this valve shaft abutting member 30, the valveshaft 13 is made to follow a movement in the axial direction of theabove mentioned motor shaft 29 to thereby open/close the above mentionedvalve 11.

A reference numeral 31 denotes a covering member mounted on an endsurface of the valve shaft abutting member 30 side in the motor case 22,32 denotes a spring retaining member mounted on a tip side of the valveshaft abutting member 30, 33 denotes a spring for urging the valve,which is hung between the spring retaining member 32 and the abovementioned covering member 31, and this spring 33 urges the valve 11 atthe tip of the valve shaft 13 via the valve shaft abutting member 30 ina direction that opens the valve 11. A reference numeral 14 denotes aspring receiver mounted on the top end of the valve shaft 13 and betweenthe spring receiver 14 and the valve body 1 is provided a return spring18.

A reference numeral 35 denotes a power source side case mounted on anopening end portion of the motor case 22. This power source side case 35is an integrally molded part made of synthetic resin and constitutes amain device body for passing a current through the brushless servo motor21 and is an integral combination of a board 37 for mounting halldevices 36 and an input/output connector part 38 having a connectorterminal 38 a. A reference numeral 39 denotes a magnet that is mountedon an end of the rotor so as to move opposite to the hall device 36.

FIG. 3 is a control circuit diagram of the brushless servo motor 21 inthe prior art. The control circuit has hall devices 36U, 36V, 36W fordetecting a position of the rotor 23, a switching unit 40 that includesa group of transistors for receiving the signals of these hall devicesand switching the passing of a current through coils 28U, 28V, 28W, andpasses the current through the coils 28U, 28V, 28W sequentially tothereby rotate the rotor 23 in a given direction.

The above mentioned switching unit 40 has switching devices (hereinafterreferred to as switch devices) 41U, 41V, 41W that receive output signalsof the hall devices 36U, 36V, 36W to be brought into conduction; aninterrupting section 42 that receives output signals of the switchdevices 41U, 41V, 41W to output an interrupting signal; a signalprocessing block 43 that receives and processes output signals of theabove mentioned hall devices 36U, 36V, 36W based on the output signal ofthe interrupting section 42; switch devices 44 to 49 that receive theoutput signals of the signal processing block 43 to be brought intoconduction; and pairs of switch devices 50 and 51, 52 and 53, 54 and 55each pair of which are connected in series to each other, have one endof one of coils 28U, 28V, 28W connected to their connection midpoint,and change a state of conduction based on the output signal of theswitch devices 44 to 49.

The above mentioned signal processing block 43 has a digital input port61 for receiving the output signals of the hall devices 36U, 36V, 36W; acounter 62 for reading in an output of the digital input port 61; a PIcontrol computing section 64 for performing a PI control computationbased on an output signal of the counter 62 and an output signal of atarget value determining section 63; an excitation duty computingsection for performing an excitation duty computation based on an outputsignal of the PI control computing section 64; an excitation phasecomputing section 66 for computing an excitation phase based on anoutput signal of the excitation duty computing section 65 and an outputsignal of the digital input port 61; a digital output port 67 foroutputting an ON signal based on an output signal of the excitationphase computing section 66; and a PWM output port 68 for outputting aPWM signal based on an output signal of the excitation phase computingsection 66 and an output signal of the excitation duty computing section65.

Next, an operation will be described.

The target value determining section 63 determines a target value. Whenthe output signal of the hall devices 36U, 36V, 36W detecting theposition of the rotor 23 brings any one of the switch devices 41U, 41V,41W into conduction, an output signal of the interrupting section 42receiving the conduction signal makes the digital input port 61 and thecounter 62 start receiving an input signal.

The PI control computing section 64 performs the PI control computationbased on a present value inputted via the counter 62 and a target valuefrom the target value determining section 63, and the excitation dutycomputing section 65 performs an excitation duty computation based on aresult of the PI control computation and outputs a result of theexcitation duty computation to the excitation phase computing section 66and the PWM output port 68.

The excitation phase computing section 66 performs the excitation phasecomputation based on the given result of the excitation duty computationand the input signal received via the digital input port 61, and outputsa result of the excitation phase computation to the digital output port67 and the PWM output port 68.

The digital output port 67 outputs an ON signal, for example, to aterminal U based on the output signal of the excitation phase computingsection 66 to bring the switch device 50 into conduction via the switchdevice 44 to thereby connect a (+) terminal of the brushless servo motor21 to a power applying terminal V. On the other hand, the PWM outputport 51 outputs an output signal, for example, to a terminal W based onthe output signals of the excitation duty computing section 65 and theexcitation phase computing section 66 to bring the switch device 55 intoconduction via the switch device 49 to thereby pass a current in adirection shown by an arrow through the brushless servo motor 21.

In this manner, the brushless servo motor 21 is operated to move downthe motor shaft 29 by a rotation of its rotor 23 to move the valve shaft13 in the same direction against the return spring 18 by this downmovement, thereby moving the control valve 11 to a target position toopen the control valve 11. Then, the hall devices 36U, 36V, 36W detectthe position of the rotating rotor 23 sequentially and switch a passingof the current through the coils 28U, 28V, 28W to rotate the rotor 23 inthe same direction. Then, power required to hold the control valve 11 atthe position against a restoring force of the return spring 18 issupplied to the brushless servo motor 21 when the control valve 11 movesnear to the target position and a difference between the presentposition and the target value becomes nearly equal to zero.

As described above, the EGR valve is given a predetermined return torquein the direction that closes the control valve 11 by the return spring18 as an urging means and is given a variable motor torque in thedirection that opens the control valve 11 by a rotational torque of thebrushless servo motor 21 in a direction against a return torque, therebyholding a state of an expected opening by a balance between thesetorque.

In this arrangement there may be a case where though the coils 28U, 28V,28W are mounted at 120 degree intervals on the motor, the hall devices36U, 36V, 36W for detecting the rotational position of the rotor 23 arenot mounted at 120 degree intervals because of a deviation caused by aflow of mounting solder or the like. FIG. 4 shows torque curves in therespective exciting states when the same current flows in U to V and Uto W, and in a case where a current flowing from of the coil U to thecoil V is changed to a current flowing from the coil U to the coil W,when the current is changed at a point t1, a continuous torque can beproduced, but when the current is changed at a delayed point t2, thetorque is greatly varied. As a result a movement of the control valve 11which moves in proportion to time, is made greatly varied near a “pointa” as shown in FIG. 4(b).

Because the brushless servo motor in the prior art is constituted in themanner described above, when this brushless servo motor is used as adriving source of the EGR valve, in a case where the control valve isheld at a predetermined opening position by the torque balance betweenthe return torque and the driving torque of the brushless servo motor21, there is presented a problem that the torque balance is madeunstable by a deviation in detecting the position of the rotor by thehall devices and a condition of load applied to the rotor.

The present invention has been made to solve the above mentionedproblem, and an object of the present invention is to stabilize torqueat a predetermined opening (middle holding) position in a brushlessservo motor.

DISCLOSURE OF THE INVENTION

A torque stabilizing device of a brushless servo motor in accordancewith the present invention includes: a brushless servo motor for drivinga driven body; a hall device of each phase for detecting a position ofpermanent magnet rotor of the brushless servo motor; a switching unitthat receives a detection signal of the hall device to switch thepassing of a current through a coil of each phase; a PI controlcomputing section for performing a PI computation based on a detectionvalue of the hall device and a predetermined target value; and anexcitation duty computing section for outputting a power source controlsignal for the brushless servo motor based on an output signal of the PIcontrol computing section, characterized by that a load directiondetecting section that detects the driven body moving near to a targetposition and outputs a signal for stopping switching phase of a currentpassing through the brushless servo motor to stop switching the phase ofthe current.

By this arrangement it is made possible to stabilize torque at apredetermined position and to hold the control valve stably at a targetopening position even if a torque balance is made unstable by adeviation in detecting the position of the rotor by the hall device or astate of load applied to the rotor.

The load direction detecting section of the torque stabilizing device ofa brushless servo motor in accordance with the present invention detectsthe driven body moving near to the target position by an inversion ofpolarity of the output signal of the PI control computing section.

By this arrangement it is made possible to surely check that the controlvalve is near the predetermined opening position and to switch currentpassing through a coil without fail and thus to hold the control valveat a target opening position stably.

The load direction detecting section of the torque stabilizing device ofa brushless servo motor in accordance with the present invention holds aphase of a current passing in a direction that the driven body is pushedby a load.

By this arrangement it is made possible that the rotational force of themotor against a return torque in vicinity of the predetermined openingposition to thereby hold the control valve at the target openingposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic explanatory diagram of an engine exhaust system.

FIG. 2 is a longitudinal cross sectional view to show a constitution ofan EGR valve.

FIG. 3 is a control circuit diagram of a brushless servo motor in theprior art.

FIG. 4 shows a waveform of a current passing through a brushless servomotor.

FIG. 5 is a control circuit diagram of a brushless servo motor inaccordance with embodiment 1.

BEST MODE FOR CARRYING OUT THE INVENTION

In order to describe the invention in more detail, a best mode forcarrying out the invention will be described based on the accompanieddrawings.

EMBODIMENT 1

FIG. 5 is a control circuit diagram of a brushless servo motor 21 inaccordance with embodiment 1. A reference numeral 71 denotes a loaddirection detecting section for checking whether a control valve 11 isin the vicinity of a predetermined opening position. Because a result ofthe PI control computation by a PI control computing section 64 remains“+” when the control valve 11 is within a predetermined opening positionbut changes to “−” when the control valve 11 exceeds the predeterminedopening position, it can be made sure that the valve is in the vicinityof the predetermined opening position by means that the load directiondetecting section 71 detects a point where the result of the PI controlcomputation changes from “+” to “−”. At this point the otherconstitution of this embodiment is quite the same as those of the priorart shown in FIG. 3 described above, so the same parts are denoted bythe same reference symbols and their further descriptions will beomitted.

Next, an operation will be described.

The target value determining section 63 determines a target value. Theoutput signal of hall devices 36U, 36V, 36W which detect the position ofthe rotor 23, brings any one of switch devices 41U, 41V, 41W intoconduction and an output signal of the interrupting section 42 receivinga signal of conduction makes the digital input port 61 and the counter62 start receiving an input signal.

The PI control computing section 64 performs the PI control computationbased on a present value received via the counter 62 and the targetvalue from the target value determining section 63 and the excitationduty computing section 65 performs an excitation duty computation basedon the computation result and outputs its computation result to theexcitation phase computing section 66 and the PWM output port 68.

The excitation phase computing section 66 performs an excitationcomputation based on the given result of the excitation duty computationand an input signal received via the digital input port 61, and outputsits computation result to the digital output port 67 and the PWM outputport 68.

The digital output port 67 outputs an ON signal, for example, to theterminal U based on an output signal of the excitation phase computingsection 66 to bring the switch device 50 into conduction via the switchdevice 44 to thereby connect the (+) terminal of the brushless servomotor 21 to the power applying terminal V. On the other hand, the PWMoutput port 68 outputs an output signal, for example, to the terminal Wbased on the output signals of the excitation duty computing section 65and the excitation phase computing section 66 to bring the switch device55 into conduction via the switch device 49 to thereby pass a current ina direction shown by an arrow through the brushless servo motor 21.

In this manner, the brushless servo motor 21 is operated to move downthe motor shaft 29 by the rotation of its rotor 23 to move the valveshaft 13 in the same direction against the return spring 18 by this downmovement, thereby moving the control valve 11 to the target position toopen the control valve 11. Then, the hall devices 36U, 36V, 36W detectthe position of the rotating rotor 23 sequentially and switch thepassing of the current through coils 28U, 28V, 28W to rotate the rotor23 in the same direction, thereby moving the control valve 11 to thevicinity of the target position. Then, when the control valve 11 ispushed in the vicinity of the target position by the load detected bythe load direction detecting section 71 based on the computation valueof the PI control computing section 64, a current is passed through thecoil phase producing a torque curve increasing torque. As a result,stabilizing of the torque can be attained.

What is claimed is:
 1. A torque stabilizing device of a brushless servomotor comprising: a brushless servo motor for driving a driven body; ahall device of each phase for detecting a position of permanent magnetrotor of the brushless servo motor; a switching unit that receives adetection signal of the hall device to switch the passing of a currentthrough a coil of each phase; a PI control computing section forperforming a PI computation based on a detection value of said halldevice and a predetermined target value; and an excitation dutycomputing section for outputting a power source control signal for saidbrushless servo motor based on an output signal of the PI controlcomputing section, characterized by that a load direction detectingsection that detects said driven body moving near to a target positionand outputs a signal for stopping switching phase of a current passingthrough said brushless servo motor to stop switching the phase of thecurrent.
 2. The torque stabilizing device of a brushless servo motoraccording to claim 1, characterized by that the load direction detectingsection detects the driven body moving near to the target position by aninversion of polarity of the output signal of the PI control computingsection.
 3. The torque stabilizing device of a brushless servo motoraccording to claim 1, characterized by that the load direction detectingsection holds a phase of a current passing in a direction that thedriven body is pushed by a load.